The present invention relates to a novel process and composition of low toxicity for the treatment of aluminum parts to produce surfaces, which can be adhesively bonded to other parts to produce durable joints.
The conventional procedure for preparing aluminum surfaces for adhesive bonding comprises etching the aluminum with an aqueous solution of sulfuric acid and sodium dichromate. The use of such etching solutions results in the contamination of the air in the etching room with fumes which are toxic and carcinogenic to workers. Equipment required to remove toxic fumes evolved during the etching process is expensive and often ineffective if accidental spills or equipment breakdowns occur. Spent chromate containing etchants and rinse waters must be treated chemically to reduce the chrominum to the trivalent state, then neutralized to precipitate the chrome and pumped to settling ponds where the chrome settled out as a sludge.
U.S. Pat. No. 4,100,015 discloses an improved etching bath for aluminum, which consists essentially of a mixture of nitric acid, sodium sulfate and ferric sulfate, and preferably also sulfuric acid. The bath contains no chromate but yet produces aluminum surfaces, which when adhesively bonded provide joint strengths comparable to those obtained using the standard chromate etch. However, while the bath reduces or eliminates the pollution and toxicity problems associated with the chromate bath, it leaves something to be desired, since during the etching of the metal the nitric acid evolves fumes including oxides of nitrogen, which are excessively toxic.